Ion beam apparatus and a method for neutralizing space charge in an ion beam

ABSTRACT

A source of thermionic electrons is provided inside the flight tube of a magnet, especially an analysing magnet, and extends along the beam flight path. This allows space charge to be neutralised along the beam&#39;s axis in spite of severely restricted electron mobility in this direction owing to the presence of substantially transverse magnetic field. Thermionically emitted electrons may contribute directly to the neutralisation of space charge in positive ion beams, or, in the case of negative ion beams, indirectly by ionizing residual or deliberately introduced neutral gas atoms or molecules. Examples are described and claimed in which the source is arranged outside the nominal beam envelope in the flight tube, but linked to the beam by magnetic flux generated in the flight tube. This reduces erosion of the source by the beam and so reduces beam contamination. In these examples, an important feature is the provision of electron repellers to reflect electrons back and forth across the beam. Alternative arrangements are described and claimed in which the source is positioned inside the beam. The thermionic electron source may comprise an array of filaments, and preferably is negatively biased with respect to the flight tube. Adjustment of this bias enables the energy of emitted electrons to be controlled.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/GB 99/01500 which has an Internationalfiling date of May 12, 1999, which designated the United States ofAmerica.

The present invention is concerned with neutralising space charge in ionbeams travelling through regions of applied magnetic field, and inparticular, although not exclusively, with neutralising space charge inan ion beam as it travels through the flight tube of an analysingmagnet.

An analysing magnet generates a substantially uniform magnetic field inits flight tube, causing an ion travelling through the flight tube tofollow a curved path in a plane perpendicular to the direction of themagnetic field. The radius of the curved path is given by:

r=mv/qB=(2Em)^(½) /qB

where v,E,m and q respectively are the velocity, kinetic energy, massand charge of the ion, and B is the magnitude of the magnetic fluxdensity in the flight tube. The analysing magnet can therefore be usedto resolve spatially (in a dispersion plane perpendicular to themagnetic field in the flight tube), ions in a beam according to theirenergy, mass and charge.

In ion implanters, an analysing magnet is used in conjunction with aselection slit to select ions of the required species from an incidentbeam for implantation in a target semiconductor substrate. Typically,the incident beam will comprise ions having substantially the sameenergy, and the magnet is arranged to focus those ions having thedesired mass/charge ratio at the selection slit so that only they passthrough the slit and go on to impinge on the target.

In spectrometry applications, analysing magnets are used to resolve ionsin a beam according to their mass, energy and charge for separatedetection.

Ideally, for both ion implantation and spectrometry applications, ionsin the beam entering the flight tube having the same energy, charge andmass should all be focused by the analysing magnet onto a common line,perpendicular to the dispersion plane, as they exit.

However, in the absence of any neutralising effect, a beam containingonly ions of a particular polarity will experience space charge effects.The mutual repulsion of the ions in the beam tends to cause the beam todiverge or “blow up”. This mutual repulsion means that the position atwhich an ion exits the magnet is no longer solely determined by itsincident velocity, mass and charge, and the applied magnetic field.

In ion implantation applications, although an incident ion may have thedesired mass/charge ratio, because of space charge effects it may not befocused on the selection slit and so may not reach the target. Thisreduces the ion implantation current reaching the target from a givensource and increases the process time required to achieve a desiredimplantation dose. In addition, space charge effects may result in anincreased number of incident beam ions hitting the sides of the flighttube. This further reduces the implantation current reaching the targetand can result in contamination of the target by particles sputtered offthe flight tube.

Similarly, in spectrometry applications, beam “blow up” inhibits thespatial resolution of different ions and reduces signal intensity.

In applications where a scanning magnet is used to deflect an ion beam(for example to scan the beam across a target) beam blow up inside themagnet is also undesirable. It reduces beam intensity and controlaccuracy.

The effect of beam space charge is especially severe for relatively lowenergy beams (e.g. 1-2 keV)since for the same beam current there is ahigher density of ions in a low energy beam.

In regions of zero electric field, such as the flight tube of ananalysing or scanning magnet, self neutralisation of ion beams tends tooccur through the production of electrons and positive ions resultingfrom collisions between beam ions and atoms of residual gas in thevacuum chamber through which the beam is passing. However, this selfneutralisation may be insufficient adequately to reduce beam blow up.This is particularly true for relatively low energy ion beams, as, atlow energies, the cross sections for electron production duringinteraction between the beam ions and residual gas atoms are extremelysmall.

Also, some self neutralisation may occur as a result of local electronproduction from the beam striking the inside of the flight tube alongthe entire beam path through the magnet. Again however, this may beinsufficient to reduce beam blow up to acceptably low levels, and in anycase is to be avoided as it results in beam loss and unwanted particlegeneration.

In regions of zero electric field and zero magnetic field (i.e. “driftspace”) a known technique to neutralise space charge is to flood theregion through which the ion beam travels with low energy (typically afew eV) electrons or ions, produced, for example, in a plasma chamberadjacent to the beam flight path. In this drift space the electrons orions are mobile and can move along and across the beam to minimise beampotential.

In regions of applied magnetic field however, the magnetic fieldseverely limits the mobility of these electrons or ions. In appliedfields of sufficient magnitude to deflect beams of ions with energies inexcess of, say, a few keV, low energy charged particles, and electronsin particular (with their small mass,) will follow paths having circularprojections of very small radii on a plane perpendicular to thedirection of applied field. In effect, the electrons are restricted justto following the magnetic field lines. They have substantially zeromobility perpendicular to the applied field, which in the case ofanalysing magnets or scanning magnets means that the electrons havesubstantially zero mobility along the beam axis.

In regions where the electric field is nominally zero, such as insidethe earthed flight tube of an analysing magnet, there may in fact be asmall amount of electron motion perpendicular to the direction ofapplied magnetic field owing to the presence of various small electricfields such as those resulting from the beam itself. This motion isknown as E×B motion. However, in such nominally E-field-free regions,electron mobility along the beam is, in general, very restricted.

Thus it is not possible to neutralise beam space charge inside regionsof applied field by introducing low energy charged particles to adjacentdrift space regions as the charged particles will not be able to migrateinto the beam.

According to a first aspect of the present invention there is providedion beam apparatus comprising:

a analysing magnet including a flight tube for receiving and conveyingthrough the magnet a beam of ions, the magnet being operable to generatea substantially uniform magnetic field in the flight tube to deflectbeam ions according to their mass/charge ratio in a dispersion planeperpendicular to the direction of said uniform magnetic field; and

a thermionic electron source inside the flight tube, arranged adjacentand outside a nominal cross section of a beam of ions travelling throughthe magnet and extending along a nominal flight path of a beamtravelling through the magnet,

the thermionic electron source being further arranged such that theprojection of the thermionic electron source on the dispersion plane andthe nominal projection on the dispersion plane of an ion beam travellingthrough the magnet overlap at a plurality of positions along the nominalflight path of the beam.

Thus, magnetic flux generated by the magnet may link the thermionicelectron source and a beam of ions travelling through the magnet at aplurality of positions along the flight path, and electrons may beemitted thermionically from the source into the beam at these positions.

This enables space charge to be neutralised at a plurality of positionsalong the beam in spite of low electron mobility along the beam, and soreduces “blow up”, keeping the beam tight. The thermionic electronsource is positioned outside the nominal beam envelope to reduce beamcontamination resulting from sputtering off the source and theassociated erosion of the source.

In the presence of the magnetic field generated by the analysing magnet,electrons emitted from the thermionic electron source will simply followfield lines passing through the source. These field lines will beperpendicular to the dispersion plane of the magnet. Only those emittedelectrons travelling along field lines which also link the ion beamwill, therefore, enter the beam and contribute to neutralisation of itsspace charge (either directly, in the case of a beam of positive ions,or, in the case of a beam of negative ions, by colliding with residualor deliberately introduced gas atoms or molecules to produce positiveions).

Clearly, the greater the degree of overlap between the projections onthe dispersion plane of the thermionic electron source and the ion beam,the greater the proportion of electrons emitted from the source that areable to enter the beam.

Advantageously, the thermionic electron source may extend substantiallyin a plane spaced from and parallel to the dispersion plane. In ananalysing magnet with a nominally horizontal dispersion plane, thethermionic electron source may therefore be arranged in a plane justabove, or just below the nominal beam cross section.

In a preferred embodiment the thermionic electron source is positionedas close as possible to the nominal beam cross section to maximise theproportion of the flight tube cross section available for beamtransport.

In certain embodiments, the thermionic electron source comprises alongitudinal electrically conductive filament running along the nominalflight path, above and substantially parallel to the nominal centre ofan ion beam travelling through the analysing magnet. The filament maytherefore be linked to the beam by magnetic flux along a substantialpart of its length, and so can be used to emit electrons thermionicallyto neutralise space charge along a large fraction of the beam's pathlength through the magnet.

A plurality of spaced-apart longitudinal filaments may be used toneutralise space charge both along the length and across the width of abeam.

In a further preferred embodiment, the thermionic electron sourcecomprises a plurality of transverse filaments, spaced along the flightpath, and each extending across the nominal beam width.

The transverse filaments may for example, be connected to bus bars atopposite sides of the beam, or may be in the form of hairpins, connectedto two separate bus bars at the same side of the beam. Hairpin geometryprovides the advantage that the filaments are less prone to breaking asa result of relative movement of the bus bars, or thermal expansion orcontraction.

The transverse filaments provide the advantage of enabling space chargeto be neutralised across the beam at a plurality of locations along itspath through the magnet.

The thermionic electron source may comprise an array or grid offilaments, which may be planar and, for example, be machined or cut froma graphite sheet.

Heat shields may be incorporated between the thermionic electron sourceand the inside of the flight tube to reduce the power needed to maintainthe source at a temperature sufficient to yield a desired emissioncurrent.

A wide variety of materials may be used for the filament orfilaments,including, for example, tungsten.

In a preferred embodiment however, graphite filaments are employed. Forion implantation applications in particular, graphite filaments aredesirable because carbon sputtered off the filaments as a result of beamstrike is a more tolerable beam contaminant than are metal atoms orions.

In cases where the beam travelling through the magnet comprises justpositive ions, the thermionically emitted electrons may neutralise thebeam space charge directly. The electrons travel along flux lines intothe beam, and once inside contribute to space charge neutralisation.

In the case of beams comprising negative ions travelling through themagnets, thermionic electrons may be used to ionise residual gas atomsor molecules in the partial vacuum of the flight tube. The positive ionsproduced by collisions between the thermionic electrons and the residualatoms or molecules may then contribute to the neutralisation of beamspace charge.

To facilitate the neutralisation of negative ion beams, charge-neutralatoms or molecules may deliberately be introduced into the flight tubefor ionisation by thermionically emitted electrons.

A thermionically emitted electron entering the beam will, in general,only remain in the beam if it suffers a collision inside the beam, andas a result loses energy. Otherwise, it will have sufficient energy toescape any beam potential well and will cease to contribute to beamspace charge neutralisation. Collisions with ions or neutral atoms maybe elastic or inelastic. At very low energies, below the thresholds forexcitation and ionisation, elastic collisions, which involve very smallenergy transfers, dominate and the electrons may simply bounce off theircollision partners and escape from the beam. Collisions with otherelectrons would involve greater energy transfer, but the collisioncross-section is very small. The inelastic collision cross-sections canbe increased significantly by increasing the electron energy by applyinga negative bias to the thermionic electron source. In preferredembodiments therefore, the thermionic electron source is negativelybiased with respect to the flight tube, and this bias voltage may beadjusted to control the energy of the emitted electrons.

This bias enables the injection of electrons into the beam to becontrolled and also provides the energy required to ionise thebackground (i.e. residual) gas which is important in the case of thespace charge neutralisation of negative ion beams. Control of electronemission energy is also important in the case of positive ion beams inthat slow positive ions produced by collisions between residual, ordeliberately introduced, neutral gas atoms or molecules and low energyemitted electrons are an important feature of space chargeneutralization because they help to trap electrons. If slow negativeions are produced, an unlikely process in most gases, they too wouldassist in the neutralization of the positive ion space charge.

A beam of positive ions may be surrounded by a sheath of negative spacecharge which provides a potential barrier to the entry of thermionicallyemitted electrons into the core of the beam. Biasing the source canprovide the emitted electrons with sufficient energy to penetrate thisbarrier.

Depending on the particular application, optimum biases in the range 3Vto70V have been found, although optimum values outside this range may beexpected.

Advantageously, one or more electron repellers, may be arranged insidethe flight tube and outside the nominal beam envelope to reflectelectrons escaping the beam back into the beam. The electron repellersare negatively biased with respect to the thermionic source, and thebias may be adjustable. In one application, a weak optimum bias of 3Vwas found. The reflectors may be positioned on opposite sides of thebeam, and enable the lifetime of electrons in the beam volume to beextended. Energetic electrons which, in the absence of collisions, wouldnot remain in the beam, may be reflected back and forth substantiallyalong field lines linking opposing repellers. The “confinement” ofelectrons between opposing repellers is possible because the mobility ofelectrons in directions perpendicular to the applied magnetic field isseverely restricted.

The repellers may be planar, and may comprise graphite sheets. Therepellers may in fact also function as heat shields.

According to a second aspect of the present invention there is providedion beam apparatus comprising:

an analysing magnet including a flight tube for receiving and conveyingthrough the magnet a beam of ions, the magnet being operable to generatea substantially uniform magnetic field in the flight tube to deflectbeam ions according to their mass/charge ratio in a dispersion planeperpendicular to the direction of said uniform magnetic field; and

a thermionic electron source inside the flight tube, extending along thenominal flight path of said beam and being arranged inside the nominalenvelope of said beam.

The term “envelope” is used to denote the three-dimensional surfacenominally bounding the beam, i.e. it represents the nominal extent ofthe beam. An object placed inside the nominal beam envelope will,therefore, nominally be struck by the beam. An object inside the nominalbeam envelope is in the nominal path of the beam.

By positioning the source at least partially inside the beam, low energyelectrons may be emitted which have insufficient energy to escape thebeam potential well. These electrons may therefore remain in the beamfor a significant time and contribute to space charge neutralisation.Again, the emitted electron energy may be controlled by varying the biason the source with respect to the flight tube.

The source inside the beam may comprise an array of filaments, which maybe planar or may follow a curved surface including the nominal beamcentre line.

The array may extend over at least part of the height and/or the widthof the beam to provide more homogeneous space charge neutralisation.

The array may comprise straight, curved, hair-pin or other geometryfilaments, and may be a grid.

The source may be electrically conductive, may comprise carbon, and/ormay comprise refractory material. The source may be arranged to beheated electrically, or by the beam, or both.

In order to reduce contamination of the beam conveyed to the remainderof the apparatus, downstream of the magnet, the parts of the thermionicelectron source inside the nominal beam envelope (i.e. these partsnominally subjected to beam strike) may be arranged to be out of line ofsight with the selection slit at the exit of the flight tube.

According to a third aspect of the present invention there is providedion beam apparatus comprising:

a magnet arranged to generate a magnetic field in a flight tube todeflect a beam of ions travelling through said flight tube; and

a thermionic electron source inside said flight tube, extending alongthe nominal flight path of said beam and arranged to emit electronsthermionically into said beam.

The source may be arranged outside the nominal beam envelope and furtherarranged to be linked to the beam at a plurality of positions along itspath by magnetic flux generated by the magnet, or the source may bearranged at least partially inside the nominal beam envelope. The sourcemay comprise an array of filaments, and the apparatus may furthercomprise electron repellers, heat shields, and means for biasing thesource and the repellers.

According to a fourth aspect of the present invention there is provideda method of neutralising space charge in a beam of ions travelling in aflight tube through a magnet, the method comprising the steps of:

providing a source of thermionic electrons inside the flight tube,adjacent and outside the nominal cross section of the beam and extendingalong the nominal flight path of the beam;

arranging the thermionic electron source to be linked to the beam at aplurality of positions along the nominal flight path by magnetic fluxgenerated by the magnet; and

emitting electrons thermionically from the thermionic electron source.

According to a fifth aspect of the present invention there is provided amethod of neutralising space charge in a beam of ions travelling in aflight tube through a magnet, the method comprising the steps of:

providing a source of thermionic electrons inside the beam, said sourceextending along the beam; and emitting electrons thermionically fromsaid source.

The magnet may, for example, be an analysing magnet, a scanning magnet,or a magnet arranged to deflect the beam for other purposes. It will beapparent that the term “flight tube” is used simply to denote the regionof space inside the magnet through which ion beam travels. The “flighttube” in general need have no sides as such, but typically comprises anelectrically conductive tube inside which the electric field is arrangedto be zero.

Embodiments of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a schematic plan view of a first embodiment of the presentinvention:

FIG. 2 is a schematic perspective view of part of the embodiment shownin FIG. 1;

FIG. 3 is a schematic plan view of a further embodiment;

FIG. 4 is a schematic plan view of an embodiment of the presentinvention;

FIG. 5 is a schematic cross section of part of the embodiment shown inFIG. 4;

FIG. 6 is a schematic cross section of part of a further embodiment;

FIG. 7 is a schematic diagram of part of the embodiment of FIG. 6;

FIG. 8 is a schematic perspective view of part of a further embodiment;and

FIG. 9 is a schematic diagram of the cross section of part of anembodiment of the present invention.

A first embodiment will now be described with reference to FIG. 1 whichshows a highly schematic plan view of an analysing magnet 1 generating asubstantially uniform magnet flux density B throughout its flight tube2. In this example the direction of the magnetic flux B is into andperpendicular to the plane of the paper and so the dispersion plane isparallel to the plane of the paper. A beam 3 of positive ions, allhaving substantially the same energy, enters the flight tube 2 and isdeflected by the magnetic field. In this example the beam is slightlydivergent as it enters the flight tube, and the analysing magnet isarranged to focus only those ions in the beam that have the desiredmass/charge ratio on a selection slit 11 in a selection electrode 10positioned downstream of the exit from the flight tube. The nominalprofile of the path through the magnet of the desired ions from theincident beam 3 is shown bounded by broken lines 31. As the figure is aplan view, this nominal profile 31 is also the nominal projection on thedispersion plane of the path of the desired component of the incidentbeam.

Ions in the incident beam which have different mass/charge ratios willfollow curved paths of different radii through the magnet and willstrike the walls of the flight tube 2 or the selection electrode 10.

A thermionic electron source in the form of a single electricallyconductive filament 5 is positioned inside the flight tube, above thenominal beam oath to reduce the number of ions striking the filament andsputter etching it away. In this example the filament 5 extends alongthe entire length of the beam path in the magnet and out into driftspace. In other examples, the filament may be shorter, located whollywithin the flight tube. Also, in other embodiments the filament maycomprise a plurality of straight sections, rather than being acontinuous curve.

The filament 5 is located in the flight tube above the beam by means notshown in the figure, and runs substantially parallel to the nominalcentre of the beam. In fact, in this embodiment the projection of thefilament onto the dispersion plane is coincident with the projection ofthe nominal beam centre line. A power supply 9 is connected to thefilament 5 to heat it to a temperature sufficient to cause the filamentto emit electrons thermionically. The positioning of the filament 5ensures that it is linked to the nominal beam profile 31 along itsentire length inside the flight tube 2 by magnetic flux B. A perspectiveview of a section of the filament linked by magnetic flux to the nominalbeam profile is shown schematic in FIG. 2. Also shown in the figure isthe path of an electron thermionically emitted from the filament. Theelectron follows a helical path along the magnetic flux passing throughthe filament, although the figure greatly exaggerates the radius of thecircular component of this helical motion. The electron is only able toenter the beam because this flux also links the beam; electron mobilityperpendicular to the applied field direction is almost zero. Once insidethe beam, the electron is able to provide some neutralisation of thebeam space charge.

In an embodiment similar to that shown in FIG. 1, a tungsten filamentwas suspended above the nominal median ray of a beam inside a flighttube and electrically heated to temperatures of approximately 2000° C.and higher to cause thermionic emission of electrons into the beam. Fora beam of 2 keV boron ions, as the filament temperature was increased toincrease thermionic electron emission current, an increase in peakintensity and a decrease in the width of the beam emerging from theanalysing magnet was observed, a result attributable to neutralisationof space charge in the beam by thermionically emitted electrons. It wasalso observed, however, that too high an emission current could actuallyreduce the beam current exiting the magnet, and so control of filamenttemperature is desirable to optimise space charge neutralisation.

For an incident ion beam of ¹⁰B and ¹¹B isotopes, thermionic emissionfrom a tungsten filament was found to enhance only the exiting beamcorresponding to one isotope. This effect is attributable to poorspacial overlap between the projections of the filament and the path ofions of the other isotope onto the dispersion plane.

In order to improve space charge neutralisation across the beam, aplurality of substantially longitudinal filaments may be employed. Suchan embodiment is shown in FIG. 3. In this example five substantiallyparallel filaments are located so as to be linked by the flight tubemagnetic field to the beam at a plurality of positions along its lengthand across its width. Means for heating the filaments, which may ofcourse comprise any suitable material, are provided (although not shownin the figure), as are means for supporting the filaments in position inthe flight tube.

An alternative way of providing space charge neutralisation at aplurality of positions along the beam and across its width is to employa series of substantially transverse filaments, as shown schematicallyin FIG. 4. In general, transverse filaments, and electrical connectionto the filaments in particular, are easier to engineer than longitudinalfilaments. In this embodiment, the filaments 5 are spaced from eachother along the length of the beam, and each transverse filament 5extends across the nominal beam width, connecting to bus bars 6 oneither side of the beam flight path.

The filaments are distributed along the beam and form a substantiallyplanar array. Although in the figure the filaments 5 and bus bars 6 areshown as separate components electrically connected, in alternativeembodiments the filaments and bus bars may be formed from a singlegraphite sheet.

Electrical currents are passed through the filaments by means of the busbars, to heat the filaments and cause them to emit electronsthermionically. The space charge neutralisation apparatus inside theflight tube 2 is incorporated in an assembly 50, whose cross section isshown schematically in FIG. 5. The outer casing of the assembly 50 canin fact also be referred to as a flight tube as it surrounds the ionbeam as it travels through the magnet 1. The casing 50 is electricallyconductive and earthed to screen the beam from external electric fields.The filaments 5 are negatively biased with respect to the casing 50 andflight tube 2, and this negative bias is adjustable to control theenergy of the thermionically emitted electrons.

Referring now to FIG. 5, the transverse filament 5 is fixed andelectrically connected to bus bars 6 by means of fixing screws 52 and islinked to the nominal cross section of the beam envelope 32 across itswhole width by the magnetic field in the flight tube. In this example,the transverse filaments are positioned above the nominal beam crosssection and lie in a plane spaced from and substantially parallel to thedispersion plane. The neutralising assembly 50 further comprises heatshields 51, 51 a arranged between the filaments 5 and the top and bottomwalls of the flight tube 50. These heat shields reduce the powerrequired to heat the filaments to temperatures sufficient to yield therequired emission currents.

The heat shields in this example are substantially planar graphitesheets extending along the nominal beam flight path and fully across thenominal beam width.

The inner heat shields 51 a are negatively biased with respect to thefilaments 5 and act as electron repellers, able to reflect electronstravelling away from the beam, along magnetic flux lines, back towardsthe beam. Thus, although an electron emitted from the filament may havesufficient energy to pass through the beam, the repellers may reflectthe electron back towards the beam and so increase its contribution tospace charge neutralisation.

The bias voltage applied to the inner heat shields 51 a is variable, andmay be adjusted to achieve optimum neutralisation. For one application,a weak optimum was observed with a bias voltage of −3V.

The filament 5 may be formed from any suitable material, including forexample tungsten or graphite. In ion implantation applications inparticular it may be preferable to use graphite filaments, as carboncontamination of the beam (and hence of the target substrate) caused byparts of the beam striking the filaments and sputtering off material ismuch more tolerable than metal contamination. In addition, any carbondeposits inside the apparatus resulting from sputtering off thefilaments are more easily removed, for example by oxidation andevacuation.

On the other hand, it may be desirable to use tungsten or other metalfilaments for their superior robustness.

It will be apparent that in other embodiments, a thermionic electronsource comprising a combination of transverse and longitudinal filamentsmay be employed, for example in the form of a fine wire mesh positioned“over” the beam path.

In general, the greater the spatial frequency of the filaments along andacross the beam, the greater the homogeneity of space chargeneutralisation.

FIG. 6 shows a schematic cross section of a neutraliser assembly 50 inaccordance with a further embodiment of the present invention. Thecasing of the assembly is optional, but in this example is made fromaluminium and is grounded. The casing may be integrated with the vacuumflight tube 2 or even the analyser magnet poles themselves.

Inside the casing, a plurality of substantially hairpin-like graphitefilaments 5 are spaced along the nominal beam flight path, each filamentextending in a plane parallel to the dispersion plane and across thebeam width (i.e. they are substantially transverse filaments).Connecting means 52 connect each filament 5 to bus bars 6 positioned onthe same side of the beam. The filaments form a substantially planararray.

The direction of the flight tube magnetic field is shown, and, in thearbitrary orientation of the figure, the filaments are accordinglypositioned adjacent and underneath the nominal beam cross section 32.

Outer graphite heat shields 51 b are grounded, as are the inner sideheat shields. In contrast, the inner top and bottom heat shields 51 aare negatively biased with respect to the filaments 5 to act as electronrepellers.

The use of electron repellers is an important factor in achievingappreciable beam space charge neutralisation in embodiments where thethermionic electron source is arranged outside the nominal beamenvelope.

The support posts 59 are non-conductive and may, for example, bealumina, or graphite with alumina collars.

FIG. 7 shows a highly schematic plan view of the arrangement of just thetransverse hairpin filaments 5 and bus bars 6 of the embodiment of FIG.6. As can be seen, the bus bars follow curved paths substantiallyparallel to the beam (ie. following the arc of the analysing magnet) andthe hairpin filaments are equispaced along the bus bars. In thisexample, the hairpin filaments are V-shaped, although of course othergeometries are possible. The hairpin or V-shaped geometries, where thefilaments connect to bus bars on the same side of the beam, areparticularly advantageous for graphite filaments, which are brittle.Brittle straight graphite filaments connected to bus bars on either sideof the beam have a tendency to break as a result of relative movement ofthe bus bars and/or thermal expansion effects. Hairpin geometryalleviates this problem.

In general, the greater the number of transverse filaments used, thebetter the space charge neutralisation of the beam. On an analysingmagnet with a radius of 40 cm, for example, good space chargeneutralisation can be achieved using around 20 filaments.

FIG. 8 shows part of another embodiment in which the thermionic electronsource is arranged inside the flight tube 2 (or beam vacuum chamber) andat least partially in the nominal path of the beam. The thermionicelectron source comprises an array of filaments 5, arranged in a“zig-zag” pattern between the top and bottom of the flight tube 2 andextending along the nominal centre line of the beam. The array thus liessubstantially on a curved path along the beam centre and extends overthe full “height” of the beam (the “height” being defined as the extentof the beam in a direction parallel to the applied field B).

The filaments are attached to the flight tube 2 by a series ofinsulators 60.

In this embodiment the filaments 5 are formed from refractory materialand are arranged to be heated by the beam only. In other embodiments thefilaments may be heated electrically at least in part, and may beconnected electrically in series or in parallel. In these otherembodiments, appropriate electrical feedthroughs to the filaments areprovided.

In alternative embodiments, the thermionic electron source may be a gridarranged at least partially inside the nominal beam envelope.

FIG. 9 shows a schematic cross section of part of another embodiment, inwhich the thermionic electron source comprises a substantially planararray of filaments 5 arranged across the nominal centre of the beamenvelope 31, the filaments being heated, at least in part, electricallyby a current supplied by means of bus bars 6 at the sides of the beampath. The flight tube 2 is earthed, and the array of filaments 5 isnegatively biased with respect to the flight tube to control the energyof electrons injected into the beam.

Electron repellers 51 a are arranged above and below the nominal beamenvelope 31 and are each negatively biased with respect to the filamentarray 5. All bias voltages are adjustable.

Low energy emitted electrons 70 may be confined in the beam by the beampotential well, whereas more energetic electrons 71 may escape the beambut be reflected back into it by the repellers 51 a.

It will be apparent from the preceding description that the presentinvention is not limited in its application to the neutralisation ofbeam space charge in analysing magnets, and may be applied in otherregions of applied magnetic field, which may be static or varying, forexample, in bending or scanning magnets.

What is claimed is:
 1. Ion beam apparatus comprising: an analysingmagnet including a flight tube for receiving and conveying through themagnet a beam of ions, the magnet being operable to generate asubstantially uniform magnetic field in the flight tube to deflect beamions according to their mass/charge ratio in a dispersion planeperpendicular to the direction of said uniform magnetic field; and athermionic electron source inside the flight tube, arranged adjacent andoutside a nominal cross section of a beam of ions travelling through themagnet and extending along a nominal flight path of a beam travellingthrough the magnet, the thermionic electron source being furtherarranged such that the projection of the thermionic electron source onthe dispersion plane and the nominal projection o n the dispersion planeof an ion beam travelling through the magnet overlap at a plurality ofpositions along the nominal flight path of the beam.
 2. Ion beamapparatus in accordance with claim 1, wherein the thermionic electronsource extends along the nominal flight path substantially in a planespaced from and parallel to the dispersion plane.
 3. Ion beam apparatusin accordance with claim 1, wherein the thermionic electron sourcecomprises an electrically conductive filament arranged to emit electronsthermionically and extending along the nominal flight oath andpositioned substantially parallel to the nominal centre of a beam ofions travelling through the magnet, the filament having a projection onthe dispersion plane which for at least a substantial part of the lengthof the filament overlaps the nominal projection on the dispersion planeof a beam of ions travelling through the magnet.
 4. Ion beam apparatusin accordance with claim 3 comprising a plurality of said electricallyconductive filaments arranged such that the projections of saidfilaments on the dispersion plane lie substantially parallel to eachother and are spaced apart across the width of the nominal projection ofthe beam on the dispersion plane.
 5. Ion beam apparatus in accordancewith claim 1 wherein the thermionic electron source comprises aplurality of electrically conductive filaments arranged to emitelectrons thermionically, each filament having a projection on thedispersion plane extending across at least part of the width of thenominal projection on the dispersion plane of a beam of ions travellingthrough the magnet, said filaments being spaced apart from each otheralong the nominal flight path.
 6. Ion beam apparatus in accordance withclaim 5 wherein the projection of each said filament extends fullyacross the width of the nominal beam projection.
 7. Ion beam apparatusin accordance with claim 5 wherein each said filament has asubstantially hairpin-like geometry.
 8. Ion beam apparatus in accordancewith claim 3 wherein said filament or each of said plurality offilaments comprises tungsten.
 9. Ion beam apparatus in accordance withclaim 3 wherein said filament or each of said plurality of filamentscomprises carbon.
 10. Ion beam apparatus in accordance with claim 1further comprising at least one heat shield arranged between thethermionic electron source and the inside of the flight tube.
 11. Ionbeam apparatus comprising: an analysing magnet including a flight tubefor receiving and conveying through the magnet a beam of ions, themagnet being operable to generate a substantially uniform magnetic fieldin the flight tube to deflect beam ions according to their mass/chargeratio in a dispersion plane perpendicular to the direction of saiduniform magnetic field; and a thermionic electron source inside theflight tube, extending along the nominal flight path of said beam andbeing arranged inside the nominal envelope of said beam.
 12. Ion beamapparatus in accordance with claim 11, wherein said thermionic electronsource comprises an array of filaments, said array extending along thenominal centre of said beam.
 13. Ion beam apparatus in accordance withclaim 12, wherein said array extends at least partially across thenominal height of said beam in the direction of said uniform magneticfield.
 14. Ion beam apparatus in accordance with claim 13, wherein saidarray lies substantially on a curved surface, said curved surface havingthe same projection on the dispersion plane as the nominal beam centreline.
 15. Ion beam apparatus in accordance with claim 12, wherein saidarray is substantially planar and parallel to the dispersion plane. 16.Ion beam apparatus in accordance with claim 12 wherein said arrayextends at least partially across the nominal width of said beam in adirection parallel to the dispersion plane.
 17. Ion beam apparatus inaccordance with claim 12, wherein said filaments are electricallyconductive.
 18. Ion beam apparatus in accordance with claim 17, whereinsaid filaments comprise carbon.
 19. Ion beam apparatus in accordancewith claim 12, wherein said filaments comprise refractory material. 20.Ion beam apparatus in accordance with claim 11, further comprising aselection slit arranged to admit only a selected portion of ionsconveyed through said magnet to the remainder of said apparatus, saidthermionic electron source being arranged such that no part of thesource inside the nominal beam envelope is in direct line of sight withsaid selection slit.
 21. Ion beam apparatus comprising: a magnetarranged to generate a magnetic field in a flight tube to deflect a beamof ions travelling through said flight tube; and a thermionic electronsource inside said flight tube, extending along the nominal flight pathof said beam and arranged to emit electrons thermionically into saidbeam.
 22. Ion beam apparatus in accordance with claim 21, wherein saidthermionic electron source is arranged adjacent and outside the nominalenvelope of said beam in said flight tube and is further arranged to belinked to said beam at a plurality of position along said nominal flightpath by magnetic flux generated by the magnet.
 23. Ion beam apparatus inaccordance with claim 21, wherein at least part of said thermionicelectron source is arranged inside the nominal envelope of said beam.24. Ion beam apparatus in accordance with claim 21, wherein saidthermionic electron source comprises an array of filaments.
 25. Ion beamapparatus in accordance with claim 24, herein said array issubstantially planar and perpendicular to said magnetic field.
 26. Ionbeam apparatus in accordance with claim 1, further comprising a supplyof charge-neutral atoms or molecules arranged to introduce said atoms ormolecules into the flight tube for ionisation by thermionically emittedelectrons.
 27. Ion beam apparatus in accordance with claim 1 whereinsaid thermionic electron source is arranged to have a negative biasvoltage with respect to the flight tube.
 28. Ion beam apparatus inaccordance with claim 27 wherein the negative bias voltage of saidthermionic electron source is adjustable.
 29. Ion beam apparatus inaccordance with claim 1, further including an electron repeller arrangedinside the flight tube, outside the nominal beam envelope, and betweenthe thermionic electron source and a wall of the flight tube, theelectron repeller being substantially planar and extending along thenominal flight path of the beam, the projection of the electron repelleronto a plane perpendicular to the direction of said magnetic field atleast partially overlapping the nominal projection of said beam ontosaid plane, the electron repeller being further arranged to have anegative bias voltage with respect to the thermionic electron source.30. Ion beam apparatus in accordance with claim 29, wherein the negativebias voltage of the electron repeller is adjustable.
 31. Ion beamapparatus in accordance with claim 29, wherein the electron repellercomprises a sheet of graphite.
 32. Ion beam apparatus in accordance withclaim 29, wherein the electron repeller is also a heat shield.
 33. Ionbeam apparatus in accordance with claim 29, further comprising a secondsaid electron repeller, the electron repeller and second electronrepeller being arranged on opposite sides of the beam.
 34. A method ofneutralising space charge in a beam of ions travelling in a flight tubethrough a magnet, the method comprising the steps of: providing a sourceof thermionic electrons inside the flight tube, adjacent and outside thenominal cross section of the beam and extending along the nominal flightpath of the beam; arranging the thermionic electron source to be linkedto the beam at a plurality of positions along the nominal flight path bymagnetic flux generated by the magnet; and emitting electronsthermionically from the thermionic electron source.
 35. A method inaccordance with claim 34 further comprising the step of arranging thethermionic electron source to be linked to the beam at a plurality ofpositions across the width of the beam by magnetic flux generated by themagnet.
 36. A method of neutralising space charge in a beam of ionstravelling in a flight tube through a magnet, the method comprising thesteps of: providing a source of thermionic electrons inside the beam,said source extending along the beam; and emitting electronsthermionically from said source.
 37. A method in accordance with claim34, to 36, further comprising the steps of: negatively biasing thethermionic electron source with respect to the flight tube; andadjusting the negative bias of the thermionic electron source to controlthe energy of the emitted electrons.
 38. A method in accordance withclaim 34, further comprising the step of reflecting electrons escapingthe beam back towards the beam using at least one negatively biasedelectron repeller.
 39. A method in accordance with claim 34, furthercomprising the steps of introducing charge-neutral atoms or moleculesinto the flight tube and ionising said atoms or molecules with saidthermionically emitted electrons.